1. Field of the Invention
This invention relates to a display apparatus, and more particularly to a display apparatus of the type which is built in a helmet and provides visual observation of display information from a display unit while the helmet is worn by a user.
2. Description of the Prior Art
Display apparatuses in the prior art have been incorporated in helmets for a pilots of vehicle such as an aircraft or a helicopter for displaying various types of information including the velocity of the vehicle.
An example of such a conventional display apparatus is shown in FIG. 43. Referring to FIG. 43, a tubular cathode ray tube (CRT) 101 for displaying various display information thereon is installed in a helmet (not shown). A reflecting mirror 102 is disposed adjacent an output end of the CRT 101 and reflects image light from the CRT 101 toward a lens 103 so that such image light is focused by the lens 103. A reflecting plate 104 is disposed on an output or light outgoing side of the lens 103 and reflects display light from the CRT 101 toward a visual point position (i.e., an eye of an observer wearing the helmet) so that display information transmitted from the CRT 101 by way of the reflecting mirror 102 and lens 103 may be visually observed by the observer.
The display apparatus, however, is considerably dangerous for the observer in that a high voltage is required to operate the CRT 101.
Such a display apparatus as shown in FIG. 44 is also conventionally known. The display apparatus of FIG. 44 is a modified version of the display apparatus shown in FIG. 43 in that the reflecting mirror 102 and the reflecting plate 104 of the display apparatus shown in FIG. 43 are eliminated, and instead a shield 106 of the helmet 105 is used as a reflecting element to reflect display light from the CRT 101 toward an eye E of an observer wearing the helmet 105. In the display apparatus of FIG. 44, a display image of the CRT 101 is enlarged by the lens 103 and reflected by the shield 106 so that a virtual image I of the display image can be visually observed from the visual point position E while the helmet 105 is worn by an observer. However, with the display apparatus, since the visual point position is near the shield, a display image is observed with only one eye. Accordingly, this display apparatus is has disadvantage, in addition to such possible danger arising from employment of a CRT as described above, in that an observer will be weary after use for a long period of time and the visual observability is not good.
Another display apparatus which eliminates the defect arising from disposition of a CRT in a helmet is also known and shown in FIG. 45. Referring to FIG. 45, the display apparatus includes a CRT 101 and a lens 103 which, however, are provided separately from a helmet 105 in which information is to be displayed. In particular, display information from the lens 103 is transmitted to the helmet 105 by way of an optical cable 107 which has an end thereof remote from the lens 103 introduced into the inside of a shield 106 of the helmet 105. Thus, an image of the display information is formed on an inner face of the shield 106 and visually observed by an observer.
With this display apparatus, however, since the optical cable 107 employed is comparatively low in flexibility, the optical cable 107 becomes an obstruction to the observer when the observer wears the helmet 105.
Such a display apparatus as shown in FIG. 46 has also been proposed which eliminates the drawbacks described above that arise from the fact that a visual point position is near a shield of a helmet as in the display apparatus shown in FIG. 44. Such a display apparatus is disclosed, for example, in Japanese Patent Laid-Open No. 44984/1989 and is constructed such that a user will wear a unit 110 in which two CRTs (not shown) are accommodated and display images of the CRTs are introduced into a helmet 105 from the unit 110 by way of a pair of optical cables 107. The display images of the CRTs may be projected to two locations of a shield 106 of the helmet 105. With the display apparatus, a user can combine and visually observe, with both eyes, display images reflected at the two locations of the shield 106.
With this display apparatus, however, since two CRTs are employed as display devices and the unit 110 in which the CRTs are accommodated is provided outside the helmet 105, there are drawbacks that the entire apparatus is large and that the unit 110 may become a hindrance to the user.
A further conventional display apparatus as shown in FIG. 47 eliminates the drawbacks of the conventional display apparatus described above. Referring to FIG. 47, the display apparatus includes a light emitting diode (LED) display 108 disposed on an inner face of the upper portion of a helmet 105. Display information of the LED display 108 is projected to an inner face of a shield 106 of a helmet 105 by way of a prism 109 so that it may be visually observed by an observer.
With this conventional display apparatus, however, since the display unit is located at an upper portion or alternatively at a left or right side portion of the helmet, the center of gravity of the helmet is at a comparatively high location. Consequently, an observer who wears the helmet may have an uneasy feeling, since the helmet itself is not balanced during use. Further, since a combiner of the helmet is required separately from the shield in order to obtain a desirable reflecting direction of display light and is located in the proximity of an eye of the observer, it may possibly present some risk to the observer.
Further, while display information is displayed with considerable safety since the LED display 108 is employed as a display device instead of a CRT, the display apparatus shown in FIG. 47 is disadvantageous in that the brightness of display information is comparatively low. Particularly, when the illuminance of external light is high such as in the daytime or in a counterlight condition, display information cannot be visually observed well.
Besides, with the conventional display apparatus shown in FIG. 47, a power source for driving the display device is required. If power is supplied directly to the display device of the helmet, for example, from a battery carried on a vehicle by way of a power supply cable, then such a power supply cable will become an obstruction to the driver of the vehicle. However, if an electric cell is instead installed in the helmet, then such an electric cell must be replaced at a suitable time, which is cumbersome. If a battery which can be re-charged is installed in the helmet, then such an exchanging of the battery is unnecessary. However, the helmet cannot be used while the battery is being charged.